Apoptosis is a genetically programmed cellular event that is characterized by well-defined morphological features, such as cell shrinkage, chromatin condensation, nuclear fragmentation, and membrane blebbing. Kerr et al. (1972) Br. J. Cancer, 26, 239-257; Wyllie et al. (1980) Int. Rev. Cytol., 68, 251-306. It plays an important role in normal tissue development and homeostasis, and defects in the apoptotic program are thought to contribute to a wide range of human disorders ranging from neurodegenerative and autoimmunity disorders to neoplasms. Thompson (1995) Science, 267, 1456-1462; Mullauer et al. (2001) Mutat. Res, 488, 211-231. Although the morphological characteristics of apoptotic cells are well characterized, the molecular pathways that regulate this process have only begun to be elucidated.
One group of proteins that is thought to play a key role in apoptosis is a family of cysteine proteases, termed caspases, which appear to be required for most pathways of apoptosis. Creagh & Martin (2001) Biochem. Soc. Trans, 29, 696-701; Dales et al. (2001) Leuk. Lymphoma, 41, 247-253. Caspases trigger apoptosis in response to apoptotic stimuli by cleaving various cellular proteins, which results in classic manifestations of apoptosis, including cell shrinkage, membrane blebbing and DNA fragmentation. Chang & Yang (2000) Microbiol. Mol. Biol. Rev., 64, 821-846.
Pro-apoptotic proteins, such as Bax or Bak, also play a key role in the apoptotic pathway by releasing caspase-activating molecules, such as mitochondrial cytochrome c, thereby promoting cell death through apoptosis. Martinou & Green (2001) Nat. Rev. Mol. Cell. Biol., 2, 63-67; Zou et al. (1997) Cell, 90, 405-413. Anti-apoptotic proteins, such as Bcl-2, promote cell survival by antagonizing the activity of the pro-apoptotic proteins, Bax and Bak. Tsujimoto (1998) Genes Cells, 3, 697-707; Kroemer (1997) Nature Med., 3, 614-620. The ratio of Bax:Bcl-2 is thought to be one way in which cell fate is determined; an excess of Bax promotes apoptosis and an excess of Bcl-2 promotes cell survival. Salomons et al. (1997) Int. J. Cancer, 71, 959-965; Wallace-Brodeur & Lowe (1999) Cell Mol. Life. Sci., 55, 64-75.
Another key protein involved in apoptosis is a protein that encoded by the tumor suppressor gene p53. This protein is a transcription factor that regulates cell growth and induces apoptosis in cells that are damaged and genetically unstable, presumably through up-regulation of Bax. Bold et al. (1997) Surgical Oncology, 6, 133-142; Ronen et al., 1996; Schuler & Green (2001) Biochem. Soc. Trans., 29, 684-688; Ryan et al. (2001) Curr. Opin. Cell Biol., 13, 332-337; Zörnig et al. (2001) Biochem. Biophys. Acta, 1551, F1-F37.
Alterations in the apoptotic pathways are believed to play a key role in a number of disease processes, including cancer. Wyllie et al. (1980) Int. Rev. Cytol., 68, 251-306; Thompson (1995) Science, 267, 1456-1462; Sen & D'Incalci (1992) FEBS Letters, 307, 122-127; McDonnell et al. (1995) Seminars in Cancer and Biology, 6, 53-60. Investigations into cancer development and progression have traditionally been focused on cellular proliferation. However, the important role that apoptosis plays in tumorigenesis has recently become apparent. In fact, much of what is now known about apoptosis has been learned using tumor models, since the control of apoptosis is invariably altered in some way in tumor cells. Bold et al. (1997) Surgical Oncology, 6, 133-142.
Cytokines also have been implicated in the apoptotic pathway. Biological systems require cellular interactions for their regulation, and cross-talk between cells generally involves a large variety of cytokines. Cytokines are mediators that are produced in response to a wide variety of stimuli by many different cell types. Cytokines are pleiotropic molecules that can exert many different effects on many different cell types, but are especially important in regulation of the immune response and hematopoietic cell proliferation and differentiation. The actions of cytokines on target cells can promote cell survival, proliferation, activation, differentiation, or apoptosis depending on the particular cytokine, relative concentration, and presence of other mediators.
The use of anti-cytokines to treat autoimmune disorders such as psoriasis, rheumatoid arthritis, and Crohn's disease is gaining popularity. The pro-inflammatory cytokines IL-1 and TNF play a large role in the pathology of these chronic disorders. Anti-cytokine therapies that reduce the biological activities of these two cytokines can provide therapeutic benefits (Dinarello and Abraham, 2002).
Interleukin 1 (IL-I) is an important cytokine that mediates local and systemic inflammatory reactions and which can synergize with TNF in the pathogenesis of many disorders, including vasculitis, osteoporosis, neurodegenerative disorders, diabetes, lupus nephritis, and autoimmune disorders such as rheumatoid arthritis. The importance of IL-1β in tumour angiogenesis and invasiveness was also recently demonstrated by the resistance of IL-1β knockout mice to metastases and angiogenesis when injected with melanoma cells (Voronov et al., 2003).
Interleukin 18 (IL-18) is a recently discovered member of the IL-1 family and is related by structure, receptors, and function to IL-1. IL-18 is a central cytokine involved in inflammatory and autoimmune disorders as a result of its ability to induce interferon-gamma (IFN-γ), TNF-α, and IL-1. IL-1β and IL-18 are both capable of inducing production of TNF-α, a cytokine known to contribute to cardiac dysfunction during myocardial ischemia (Maekawa et al., 2002). Inhibition of IL-18 by neutralization with an IL-18 binding protein was found to reduce ischemia-induced myocardial dysfunction in an ischemia/reperfusion model of suprafused human atrial myocardium (Dinarello, 2001). Neutralization of IL-18 using a mouse IL-18 binding protein was also able to decrease IFN-γ, TNF-α, and IL-1β transcript levels and reduce joint damage in a collagen-induced arthritis mouse model (Banda et al., 2003). A reduction of IL-18 production or availability may also prove beneficial to control metastatic cancer as injection of IL-18 binding protein in a mouse melanoma model successfully inhibited metastases (Carrascal et al., 2003). As a further indication of its importance as a pro-inflammatory cytokine, plasma levels of IL-18 were elevated in patients with chronic liver disease and increased levels were correlated with the severity of the disease (Ludwiczek et al., 2002). Similarly, IL-18 and TNF-α were elevated in the serum of diabetes mellitus patients with nephropathy (Moriwaki et al., 2003). Neuroinflammation following traumatic brain injury is also mediated by pro-inflammatory cytokines and inhibition of IL-18 by the IL-18 binding protein improved neurological recovery in mice following brain trauma (Yatsiv et al., 2002).
TNF-α, a member of the TNF family of cytokines, is a pro-inflammatory cytokine with pleiotropic effects ranging from co-mitogenic effects on hematopoietic cells, induction of inflammatory responses, and induction of cell death in many cell types. TNF-α is normally induced by bacterial lipopolysaccharides, parasites, viruses, malignant cells and cytokines and usually acts beneficially to protect cells from infection and cancer. However, inappropriate induction of TNF-α is a major contributor to disorders resulting from acute and chronic inflammation such as autoimmune disorders and can also contribute to cancer, AIDS, heart disease, and sepsis (reviewed by Aggarwal and Natarajan, 1996; Sharma and Anker, 2002). Experimental animal models of disease (i.e. septic shock and rheumatoid arthritis) as well as human disorders (i.e. inflammatory bowel diseases and acute graft-versus-host disease) have demonstrated the beneficial effects of blocking TNF-α (Wallach et al., 1999). Inhibition of TNF-α has also been effective in providing relief to patients suffering autoimmune disorders such as Crohn's disease (van Deventer, 1999) and rheumatoid arthritis (Richard-Miceli and Dougados, 2001). The ability of TNF-α to promote the survival and growth of B lymphocytes is also thought to play a role in the pathogenesis of B-cell chronic lymphocytic leukemia (B-CLL) and the levels of TNF-α being expressed by T cells in B-CLL was positively correlated with tumour mass and stage of the disease (Bojarska-Junak et al., 2002). Interleukin-1β (IL-1β) is a cytokine known to induce TNF-α production.
The amino acid sequence of eIF-5A is well conserved between species, and there is strict conservation of the amino acid sequence surrounding the hypusine residue in eIF-5A, which suggests that this modification may be important for survival. Park et al. (1993) Biofactors, 4, 95-104. This assumption is further supported by the observation that inactivation of both isoforms of eIF-5A found to date in yeast, or inactivation of the DHS gene, which catalyzes the first step in their activation, blocks cell division. Schnier et al. (1991) Mol. Cell. Biol., 11, 3105-3114; Sasaki et al. (1996) FEBS Lett., 384, 151-154; Park et al. (1998) J. Biol. Chem., 273, 1677-1683. However, depletion of eIF-5A protein in yeast resulted in only a small decrease in total protein synthesis suggesting that eIF-5A may be required for the translation of specific subsets of mRNA's rather than for protein global synthesis. Kang et al. (1993), “Effect of initiation factor eIF-5A depletion on cell proliferation and protein synthesis,” in Tuite, M. (ed.), Protein Synthesis and Targeting in Yeast, NATO Series H. The recent finding that ligands that bind eIF-5A share highly conserved motifs also supports the importance of eIF-5A. Xu & Chen (2001) J. Biol. Chem., 276, 2555-2561. In addition, the hypusine residue of modified eIF-5A was found to be essential for sequence-specific binding to RNA, and binding did not provide protection from ribonucleases.
In addition, intracellular depletion of eIF-5A results in a significant accumulation of specific mRNAs in the nucleus, indicating that eIF-5A may be responsible for shuttling specific classes of mRNAs from the nucleus to the cytoplasm. Liu & Tartakoff (1997) Supplement to Molecular Biology of the Cell, 8, 426a. Abstract No. 2476, 37th American Society for Cell Biology Annual Meeting. The accumulation of eIF-5A at nuclear pore-associated intranuclear filaments and its interaction with a general nuclear export receptor further suggest that eIF-5A is a nucleocytoplasmic shuttle protein, rather than a component of polysomes. Rosorius et al. (1999) J. Cell Science, 112, 2369-2380.
The first cDNA for eIF-5A was cloned from human in 1989 by Smit-McBride et al., and since then cDNAs or genes for eIF-5A have been cloned from various eukaryotes including yeast, rat, chick embryo, alfalfa, and tomato. Smit-McBride et al. (1989) J. Biol. Chem., 264, 1578-1583; Schnier et al. (1991) (yeast); Sano, A. (1995) in Imahori, M. et al. (eds), Polyamines, Basic and Clinical Aspects, VNU Science Press, The Netherlands, 81-88 (rat); Rinaudo & Park (1992) FASEB J., 6, A453 (chick embryo); Pay et al. (1991) Plant Mol. Biol., 17, 927-929 (alfalfa); Wang et al. (2001) J. Biol. Chem., 276, 17541-17549 (tomato).